GOI (gate oxide integrity) evaluation is very effective as a method for evaluating silicon and other semiconductor wafers (see, for example, M. Tamatsuka et al., “Medium Field Breakdown Origin on Metal Oxide Semiconductor Capacitor Containing Grown-in Czochralski Silicon Crystal Defects”, Jpn. J. Appl. Phys., Vol. 37 (1998), pp. 1236-1239), and is commonly used widely. This method makes it possible to detect COPs (crystal originated particles) in the silicon wafer or the effects of metal contamination with an extremely high degree of sensitivity.
The GOI evaluation is performed as follows. The front surface of a silicon wafer, for example, is oxidized to form a silicon dioxide film (a gate oxide film) as an insulator film, an electrode such as a polysilicon film is formed thereon, whereby a MOS capacitor having a MOS structure is fabricated, then electrical stress is applied to the electrode to break down the gate oxide film, and the quality of the silicon wafer is evaluated based on the dielectric breakdown field intensity thus observed.
For example, when a defect such as a COP is present in the main surface of a silicon wafer, a silicon dioxide film is also formed on the inner wall of a void portion of the COP at the time of formation of an oxide film on the main surface of the silicon wafer. The oxide film formed on the inner wall of the COP has a reduced thickness at a corner (a corner part) of an octahedral structure. As a result, when an electrode is formed on the oxide film and an electric field is applied, electrical stress is concentrated in the part where the thickness of the oxide film is reduced, which is believed to cause a breakdown at a low electric field intensity. Therefore, by using such a phenomenon, it is possible to evaluate a silicon wafer by detecting a COP or the like present in the silicon wafer.
The above-described application of electrical stress can be performed by the TZDB (Time Zero Dielectric Breakdown) method or the TDDB (Time Dependent Dielectric Breakdown) method.
In the TZDB method, a value of current flowing through a MOS capacitor is monitored while changing the electric field intensity from 0 MV/cm to about 15 MV/cm in a stepwise fashion, and the electric field intensity when the gate oxide film of the MOS capacitor breaks down, that is, when a breakdown occurs, is measured. An insulator film having a dielectric breakdown field intensity equal to or greater than a predetermined value, for example, 8 MV/cm or greater, is judged to be good, and otherwise it is judged to be a failure, and, based on the ratio of the number of MOS capacitors judged to be good to the total number of MOS capacitors to which a voltage is applied, the quality of the insulator film is evaluated.
On the other hand, the TDDB method is a method by which constant electrical stress is continuously applied to an insulator film, and, based on the time before a dielectric breakdown occurs, the life of the insulator film is evaluated. For example, in the TDDB method in which a constant current is applied, a constant current is continuously applied to an insulator film, changes with time are obtained by detecting the electric field intensity at a predetermined time interval, and the time before a dielectric breakdown occurs is evaluated.
Moreover, in addition to the TDDB method and the TZDB method, there are other methods by which evaluations can be performed by measuring a defect in a silicon wafer. For example, there is a measuring method called DSOD (Direct Surface Oxide Defect).
The following is a description of this method. First, for example, a thin oxide film (25 nm to 50 nm) is grown on a silicon wafer to be evaluated, and electric charges are given thereto in alcohol in which Cu ions are dissolved. Then, an oxide film containing a defect breaks down, near that broken-down spot, Cu2+ ions in alcohol combine with electrons and form metal Cu, and this metal Cu is precipitated. This makes it possible to identify the location of a weak oxide film.
This method makes it possible to detect a defect of microscopic size (size: 10 nm to 20 nm), and detect a defect with a high degree of precision even when the defect density of a silicon wafer to be evaluated is extremely low.
With these evaluation methods, previously, with the aim of improving a TDDB characteristic and a TZDB characteristic, various crystals such as a normal crystal, a slowly-cooled crystal, a quickly grown crystal, a defect-free crystal (an N-region crystal), and a nitrogen-doped crystal were formed.
In particular, an N region is a region outside an OSF region located midway between a V-rich region and an I-rich region, the region free from an FPD, an LSTD, and a COP which are caused by a vacancy and an LSEPD and an LFPD which are caused by a dislocation loop, and such an N-region crystal can be produced by adjusting a V/G value which is the ratio between a pulling rate V and a furnace temperature distribution G in the direction of a pulling shaft near the solid-liquid interface (JP-A-8-330316).
In each case, the crystals were evaluated by the conventional TDDB method or TZDB method, which can be performed with relative ease as described above. As a result, a crystal having an extremely low defect density, such as a defect-free crystal (an N-region crystal), could achieve a passing rate of 100% for the TDDB characteristic and the TZDB characteristic, which is the ultimate goal of GOI.
However, due to an extremely low production margin of an N-region crystal, even when production is performed under the conditions of production of an N-region crystal, the N-region crystal could not always be obtained. The problem is that, even when, for example, a relatively large-diameter N-region silicon wafer or the like having a diameter of 200 mm or more is set as a wafer to be evaluated and subjected to a conventional GOI evaluation, and the results reveal that a passing rate for the TDDB characteristic and the TZDB characteristic is 100%, when the same silicon wafer is evaluated by using a higher-precision defect measuring method such as the above-described DSOD method, a defect may be found. Therefore, a further improvement in precision is required.